Light Alloys- Aluminium Properties, Uses and Extraction Flashcards

1
Q

The good properties of aluminium

A

Low density (1/3 of steel)
Highly resistant to most forms of corrosion
Superb conductor of electricity (along with other intrinsic qualities mean has replaced Cu in many electrical applications)
Non-magnetic and non-combustible (invaluable for electronics or offshore structures)
Non-toxic and impervious (good for food and packaging)
Strong and ductile (along with low density mean extensive use in transport)

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2
Q

Corrosion resistance of aluminium

A

Metal’s natural coating of Al2O3 provides highly effective barrier to the ravages of air, temperature, moisture and chemical attack making Al a useful construction material

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3
Q

How abundant is Al?

A

3rd most abundant element in earth’s crust

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4
Q

Atomic number and mass

A

13 and 26.98

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5
Q

Construction industry

A

Used for external features like roofing, doors, window frames, gutters.
Also internal features like ceilings, doors, elevators, floor furniture, partition walls, signs, Venetian blinds

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6
Q

Which modes of transport use Al?

A

High speed trains and ferries, automotive (car body), aerospace.

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7
Q

How does lightweighting of cars affect CO2 emissions?

A

For every 100kg saved on mass of car this equates to reduction of 9g of CO2 per km.

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8
Q

Recyclability of Al in cars

A

95% of end-of-life Al in cars can be recycled. EU and Japan require 95% recycling of the total material used in new cars

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9
Q

Secondary weight savings in Al cars

A

When Al extensively used, secondary weight savings can exceed 50%. These are due to allowing drivetrain, motor and chassis downsizing

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10
Q

How much Al do drivetrain, chassis and suspension and car body generally use?

A

Drivetrain (engine block, radiator…) 50%.
Chassis and suspension (wheels, steering…) 30%.
Car body (bonnet, doors, bumper…) 20%

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11
Q

Use in packaging

A

Disposable food containers, drink cans, aerosol cans

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12
Q

Electrical and thermal applications

A

Pans, motors. Most overhead transmission lines are Al

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13
Q

Other application of Al

A

Furniture, accessories, art

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14
Q

Where is Al often made?

A

Where hydroelectric power is cheap

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15
Q

Where does primary Al come from?

A

Bauxite. Then goes to alumina then Al

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16
Q

Stages of Al extraction

A

Bauxite is a mixed oxide ore containing about 50% hydrated alumina. Separated from other oxides by dissolution and reprecipitation reaction with NaOH (Bayer process). Then dissolved in a molten salt mixture of cryolite (Na3AlF6) and electrolytically reduced to molten Al (Hall-Heroult process). 4kg bauxite makes 2kg Al2O3 makes 1kg Al.

17
Q

What is useful about Al for its extraction?

A

Most metals only dissolve in acid but Al also dissolves in alkali so can be selectively removed

18
Q

How much power is used to extract Al from bauxite?

A

70,000kWh consumed per tonne of Al produced from bauxite. Steel requires 13-16,000 kWh/tonne from iron ore to finished product

19
Q

Bauxite resources

A

Estimated to be 55-75 billion tons. Africa (33%), Oceania (24%), South America and Caribbean (22%), Asia (15%) and elsewhere (6%)

20
Q

How is Hall-Heroult process arranged?

A

Molten mixture of Al2O3 and cryolite (Na3AlF6) along with additives like CaF and AlF3. This is in a graphite-lined tank which is the cathode. Have graphite electrodes (anodes) inside. Liquid Al forms at bottom and exits at bottom (continuous process). Typically have 300 cells connected in series to form a potline.

21
Q

Reactions and temperatures in Hall-Heroult process

A

Overall reaction is 2Al2O3 + 3C -> 4Al + 3CO2
Forms bubbles fo CO2 at the anodes. Alumina melts at 2040°C and is poor electrical conductor. Typical melt contains 80-90% cryolite. Continuous process with bath running all the time at 950°C. Largest cells have daily output of 4tonnes

22
Q

Why does aerospace sector waste so much Al?

A

Needs large blocks for suitable thermomechanical processing for right grain structure and properties. Wastes around 80% of Al it uses. Aerospace industry relies on Al industry not the other way around